The present invention relates to a two-wavelength semiconductor laser diode array having a monolithic structure including two semiconductor laser diodes lazing at different wavelengths formed on a single substrate, and a method of fabricating the semiconductor laser diode array.
Appearance of a digital video disk (DVD) has advanced increase of the recording density of an optical disk, and an optical disk with mass storage of 8.5 GB has been realized these days. A general DVD reproducing system should reproduce data from not only a DVD but also a compact disk (CD), and is sometimes required to be capable of reproducing data from and recording data in a write-once CD (CD-R) currently rapidly spreading. As reproducing light for reproducing data from a DVD, a red laser beam of a wavelength of a 650 nm band is used, and as reproducing light for reproducing data from a CD or a CD-R, an infrared laser beam of a wavelength of a 780 nm band is used. Accordingly, a current DVD reproducing system includes two semiconductor laser chips, namely, a red semiconductor laser chip for generating the red laser beam and an infrared semiconductor laser chip for generating the infrared laser beam.
In accordance with development of compact information processing equipment such as a personal computer, it is necessary to make a DVD reproducing system more compact, and for this purpose, it is indispensable to realize a compact and thin optical pickup part. One means for realizing a compact and thin optical pickup part is simplification of an optical system. One method for simplifying an optical system is integration of a red semiconductor laser chip and an infrared semiconductor laser chip. As described above, a current DVD reproducing system includes two optical system components for the red semiconductor laser chip and the infrared semiconductor laser chip. Therefore, when the red and infrared semiconductor laser chips are integrated, the two optical system components can be shared, resulting in realizing a compact and thin optical pickup part.
As examples of the integration of a red semiconductor laser chip and an infrared semiconductor laser chip, a monolithic semiconductor laser diode array integrated on a single substrate is reported in Japanese Laid-Open Patent Publication No. 11-186651 (Conventional Example 1) and xe2x80x9cThe 60th Autumn Applied Physics Lecture Meetingxe2x80x9d, 3a-ZC-10 (Conventional Example 2).
In the semiconductor laser diode array of Conventional Example 1, each of a red laser chip and an infrared laser chip includes, as a current block (confinement) layer for efficiently injecting a current into an active layer, GaAs with an energy gap equivalent to or smaller than the energy gap (band gap) of the active layer. In this manner, a complex refractive index waveguide structure is employed, in which a laser beam emitted from the active layer is absorbed so as to effectively confine generated light within a stripe-shaped region.
In the semiconductor laser diode employing the complex refractive index waveguide structure, however, generated light is absorbed by a current block layer of GaAs, and hence, it is very difficult to attain a self-oscillation characteristic and a high-temperature high-output-power characteristic required in an optical disk unit.
Alternatively, the semiconductor laser diode array of Conventional Example 2 has the so-called gain waveguide structure including no current block layer, and hence, light is never absorbed by a current block layer. The semiconductor laser diode having the gain waveguide structure, however, does not have the complex refractive index waveguide structure for effectively confining generated light. Therefore, in order to attain low noise required in an optical disk unit, it is necessary to provide means for suppressing interference between oscillation spectra by, for example, employing multiple modes for the oscillation spectra.
Furthermore, even when the multiple modes are employed for the oscillation spectra, the half bandwidth of each spectrum is so small that emitted light and return light of the emitted light to the semiconductor laser diode can easily interfere each other. Therefore, the relative noise intensity (RIN) cannot be lowered to xe2x88x92130 dB/Hz or less as is desired in an optical disk unit. Accordingly, the semiconductor laser diode array having the gain waveguide structure of Conventional Example 2 needs means for lowering the RIN by using a 1/4 xcex plate or the like, and hence, it is difficult to reduce the number of components included in the optical pickup part. In order to solve these problems, the semiconductor laser diode array should indispensably have a self-oscillation characteristic.
In addition, although the semiconductor laser diode array with the gain waveguide structure has a current confining function, it does not have a light confining function utilizing a refractive index distribution along a direction parallel to the principal plane of the active layer. Therefore, when it is operated at low output power of 10 mW or less in reproducing data from a DVD or a CD, a single lateral mode characteristic can be kept at room temperature, but a stable lateral mode characteristic is difficult to keep at a high temperature because carriers are so largely injected that gain can be more easily attained in a higher mode. Furthermore, since it does not have a light confining mechanism, the lateral mode characteristic is more difficult to stabilize when it is operated at high output power.
Moreover, since the optical system components are shared in the conventional monolithic two-wavelength laser diode array, the active layers of the respective laser diodes are preferably placed in the same position, namely, at the same height from the substrate surface. However, although the laser diode array is monolithic, the active layers of the respective laser diodes have different compositions, and hence, the active layers should be grown through different processes. Therefore, the heights of the active layers disadvantageously differ from each other.
An object of the invention is, for solving the aforementioned conventional problems, realizing a monolithic semiconductor laser diode array having a stable self-oscillation characteristic and definitely capable of operating at high output power and a high temperature in which a difference in the height between active layers is suppressed.
In order to achieve the object, in fabrication of a monolithic semiconductor laser diode array of this invention, a first laser diode showing laser action at a longer wavelength is formed priorly to a second laser diode showing laser action at a shorter wavelength. At this point, since a cladding layer formed closer to a substrate in the second laser diode may have a smaller thickness than a cladding layer formed closer to the substrate in the first laser diode, a buffer layer for improving the crystallinity of the second laser diode is formed and the buffer layer is provided with a height adjusting function.
Furthermore, both the first laser diode and the second laser diode are provided with a refractive index waveguide mechanism, so as to realize a stable lateral mode characteristic, and a current block layer of each laser diode is formed in a real refractive index waveguide structure, so as to realize a stable self-oscillation characteristic, a high output power operation and a high temperature operation.
Specifically, the semiconductor laser diode array of this invention comprises a first laser diode including a first cladding layer formed on a substrate from a first semiconductor of a first conductivity type, a first active layer formed on the first cladding layer from a second semiconductor and a second cladding layer formed on the first active layer from a third semiconductor of a second conductivity type; and a second laser diode including a third cladding layer formed from a fourth semiconductor of the first conductivity type on the substrate with a space from the first laser diode, a second active layer formed on the third cladding layer from a fifth semiconductor having a larger energy gap than the first active layer and a fourth cladding layer formed on the second active layer from a sixth semiconductor of the second conductivity type, and the second laser diode further includes a height adjusting buffer layer formed between the substrate and the third cladding layer from a seventh semiconductor of the first conductivity type and having a thickness set for placing the second active layer at substantially the same height from a substrate surface as a height from the substrate surface of the first active layer.
In the semiconductor laser diode array of this invention, the second laser diode including the second active layer having a larger energy gap than the first active layer of the first laser diode further includes the height adjusting buffer layer whose thickness is set for placing the second active layer at substantially the same height from the substrate surface as the first active layer. Therefore, not only the crystallinity of the semiconductor layers of the second laser diode is improved but also the height from the substrate surface of the first active layer can substantially accord with that of the second active layer. As a result, a difference in the height between light emitting parts of laser beams having difference wavelengths can be suppressed.
In the semiconductor laser diode array, it is preferred that the first laser diode includes a first current block layer formed on the second cladding layer from an eighth semiconductor having a larger energy gap than the first active layer and including a stripe-shaped opening for selectively injecting carriers into the first active layer, and that the second laser diode includes a second current block layer formed on the fourth cladding layer from a ninth semiconductor having a larger energy gap than the second active layer and including a stripe-shaped opening for selectively injecting carriers into the second active layer. In this manner, the light emitted from the first and second active layers are never absorbed by the first and second current block layers, respectively, resulting in definitely realizing a self-oscillation characteristic and a high output power characteristic at a high temperature.
In the semiconductor laser diode array, it is preferred that, in the first laser diode, a difference between an effective refractive index of a region of the first current block layer included in the opening along a vertical direction to the substrate surface and an effective refractive index of a region of the first current block layer excluding the opening along the vertical direction to the substrate surface is approximately 2xc3x9710xe2x88x923 through approximately 1xc3x9710xe2x88x922, and that, in the second laser diode, a difference between an effective refractive index of a region of the second current block layer included in the opening along the vertical direction to the substrate surface and an effective refractive index of a region of the second current block layer excluding the opening along the vertical direction to the substrate surface is approximately 2xc3x9710xe2x88x923 through approximately 1xc3x9710xe2x88x922.
In the semiconductor laser diode array, it is preferred that the first current block layer is of the first conductivity type and includes arsenic and that the second current block layer is of the first conductivity type and includes phosphorus. In this manner, the first laser diode can be an infrared laser diode that is formed from a group III-V compound semiconductor including aluminum and gallium as the group III elements and arsenic as the group V element and can emit an infrared laser beam. Also, the second laser diode can be a red laser diode that is formed from a group III-V compound semiconductor including aluminum, gallium and indium as the group III elements and phosphorus as the group V element and can emit a red laser beam.
In the semiconductor laser diode array, it is preferred that the first current block layer and the second current block layer are of the first conductivity type and include phosphorus. In this manner, even when the first laser diode is an infrared laser diode including, in the first active layer and the first and second cladding layers, compound semiconductors including gallium and arsenic and the second laser diode is a red laser diode including, in the second active layer and the third and fourth cladding layers, compound semiconductors including gallium, indium and phosphorus, layers formed on the first current block layer and the second current block layer, such as contact layers, can be formed from one compound semiconductor layer including phosphorus, and hence, the fabrication can be simplified.
In the semiconductor laser diode array, it is preferred that the first current block layer and the second current block layer are of the first conductivity type and include arsenic. In this manner, even when the first laser diode is an infrared laser diode including, in the first active layer and the first and second cladding layers, compound semiconductors including gallium and arsenic and the second laser diode is a red laser diode including, in the second active layer and the third and fourth cladding layers, compound semiconductors including gallium, indium and phosphorus, layers formed on the first current block layer and the second current block layer, such as contact layers, can be formed from one compound semiconductor layer including arsenic, and hence, the fabrication can be simplified.
The first method of fabricating a semiconductor laser diode array of this invention comprises the steps of forming a first cladding layer on a substrate from a first semiconductor of a first conductivity type; forming a first active layer on the first cladding layer from a second semiconductor; forming a second cladding layer on the first active layer from a third semiconductor of a second conductivity type; forming, on the second cladding layer, a first current block layer including a stripe-shaped opening from a fourth semiconductor minimally absorbing light emitted from the first active layer; forming a third cladding layer from a fifth semiconductor of the second conductivity type over the first current block layer including the opening; forming a first semiconductor laser diode including the first cladding layer, the first active layer, the second cladding layer, the first current block layer and the third cladding layer and forming a second laser diode formation region on the substrate by etching the first cladding layer, the first active layer, the second cladding layer, the first current block layer and the third cladding layer with an area on the third cladding layer including the opening of the first current block layer masked; forming a height adjusting buffer layer on the second laser diode formation region from a sixth semiconductor of the first conductivity type; forming a fourth cladding layer on the height adjusting buffer layer from a seventh semiconductor of the first conductivity type; forming a second active layer on the fourth cladding layer from an eighth semiconductor having a larger energy gap than the first active layer; forming a fifth cladding layer on the second active layer from a ninth semiconductor of the second conductivity type; forming, on the fifth cladding layer, a second current block layer including a stripe-shaped opening extending along substantially the same direction as a longitudinal direction of the opening of the first current block layer from a tenth semiconductor minimally absorbing light emitted from the second active layer; forming a sixth cladding layer from an eleventh semiconductor of the second conductivity type over the second current block layer including the opening; and forming a second semiconductor laser diode including the height adjusting buffer layer, the fourth cladding layer, the second active layer, the fifth cladding layer, the second current block layer and the sixth cladding layer by etching the height adjusting buffer layer, the fourth cladding layer, the second active layer, the fifth cladding layer and the second current block layer and the sixth cladding layer with an area on the sixth cladding layer including the opening of the second current block layer masked.
In the first method of fabricating a semiconductor laser diode array, in forming the second laser diode including the second active layer having a larger energy gap than the first active layer of the first semiconductor laser diode, the height adjusting buffer layer is formed on the laser diode formation region exposed on the substrate by the etching. Therefore, not only the crystallinity of the second laser diode can be improved but also the height from the substrate surface of the first active layer can substantially accord with that of the second active layer. As a result, a difference in the height between light emitting parts of laser beams having different wavelengths can be suppressed.
In the first method of fabricating a semiconductor laser diode array, a difference in a height from a substrate surface to a top surface between the third cladding layer and the sixth cladding layer is preferably approximately xc2x11 xcexcm or less. In this manner, when the semiconductor laser diode array of this invention is packaged by, for example, a junction-down method, a difference in the height from the substrate to the opposite face (junction face) between the first semiconductor laser diode and the second semiconductor laser diode can be suppressed to approximately xc2x11 xcexcm or less, resulting in improving the yield in packaging.
The second method of fabricating a semiconductor laser diode array of this invention comprises the steps of forming a first cladding layer on a substrate from a first semiconductor of a first conductivity type; forming a first active layer on the first cladding layer from a second semiconductor; forming a second cladding layer on the first active layer from a third semiconductor of a second conductivity type; forming a first current block layer on the second cladding layer from a fourth semiconductor minimally absorbing light emitted from the first active layer; forming one part of a first semiconductor laser diode and forming a second laser diode formation region on the substrate by etching the first cladding layer, the first active layer, the second cladding layer and the first current block layer with a first laser diode formation region on the first current block layer masked; forming a height adjusting buffer layer on the second laser diode formation region from a fifth semiconductor of the first conductivity type; forming a third cladding layer on the height adjusting buffer layer from a sixth semiconductor of the first conductivity type; forming a second active layer on the third cladding layer from a seventh semiconductor having a larger energy gap than the first active layer; forming a fourth cladding layer on the second active layer from an eighth semiconductor of the second conductivity type; forming, on the fourth cladding layer, a second current block layer including a stripe-shaped opening from the fourth semiconductor minimally absorbing light emitted from the second active layer; forming stripe-shaped openings extending at an interval in parallel to each other by etching the first current block layer and the second current block layer; forming a ninth semiconductor layer of the second conductivity type over the first current block layer and the second current block layer including the openings; and forming the other part of the first semiconductor laser diode including a fifth cladding layer and forming a second semiconductor laser diode including the height adjusting buffer layer, the third cladding layer, the second active layer, the fourth cladding layer and a sixth cladding layer by forming the fifth cladding layer on the first current block layer from the ninth semiconductor layer and forming the sixth cladding layer on the second current block layer from the ninth semiconductor layer through etching of at least the ninth semiconductor layer with the first laser diode formation region and the second laser diode formation region on the ninth semiconductor layer masked.
In the second method of fabricating a semiconductor laser diode array of this invention, in addition to the characteristics of the first method of fabricating a semiconductor laser diode array, the stripe-shaped openings can be simultaneously formed in the first current block layer and the second current block layer of the fourth semiconductor. Specifically, both the openings can be formed through one exposure process using one mask pattern having an opening pattern corresponding to the openings of the first and second current block layers extending substantially in parallel to each other. Therefore, the distance between the openings can be determined depending upon the accuracy in the lithography. Furthermore, since both the first current block layer and the second current block layer are formed from the fourth semiconductor layer, the ninth semiconductor layer can be formed on the top surface including the openings through one growth process. Therefore, the fifth cladding layer and the sixth cladding layer can be easily formed.
In the second method of fabricating a semiconductor laser diode array, a difference in a height from a substrate surface to a top surface between the fifth cladding layer and the sixth cladding layer is preferably approximately xc2x11 xcexcm or less.
The third method of fabricating a semiconductor laser diode array of this invention comprises the steps of forming a first cladding layer on a substrate from a first semiconductor of a first conductivity type; forming a first active layer on the first cladding layer from a second semiconductor; forming a second cladding layer on the first active layer from a third semiconductor of a second conductivity type; forming a third cladding layer in the shape of a ridge on the second cladding layer from a fourth semiconductor of the second conductivity type; forming, on the second cladding layer on both sides of the third cladding layer, a first current block layer from a fifth semiconductor minimally absorbing light emitted from the first active layer; forming a first semiconductor laser diode including the first cladding layer, the first active layer, the second cladding layer, the third cladding layer and the first current block layer and forming a second laser diode formation region on the substrate by etching the first cladding layer, the first active layer, the second cladding layer and the first current block layer with an area on the first current block layer including the third cladding layer masked; forming a height adjusting buffer layer on the second laser diode formation region from a sixth semiconductor of the first conductivity type; forming a fourth cladding layer on the height adjusting buffer layer from a seventh semiconductor of the first conductivity type; forming a second active layer on the fourth cladding layer from an eighth semiconductor having a larger energy gap than the first active layer; forming a fifth cladding layer on the second active layer from a ninth semiconductor of the second conductivity type; forming, on the fifth cladding layer, a sixth cladding layer in the shape of a ridge extending along substantially the same direction as a longitudinal direction of the third cladding layer from a tenth semiconductor of the second conductivity type; forming, on the fifth cladding layer on both sides of the sixth cladding layer, a second current block layer from an eleventh semiconductor minimally absorbing light emitted from the second active layer; and forming a second semiconductor laser diode including the height adjusting buffer layer, the fourth cladding layer, the second active layer, the fifth cladding layer, the sixth cladding layer and the second current block layer by etching the height adjusting buffer layer, the fourth cladding layer, the second active layer, the fifth cladding layer and the second current block layer with an area on the second current block layer including the sixth cladding layer masked.
In the third method of fabricating a semiconductor laser diode array, in forming the second laser diode including the second active layer having a larger energy gap than the first active layer of the first semiconductor laser diode, the height adjusting buffer layer is formed on the laser diode formation region exposed on the substrate by etching. Therefore, not only the crystallinity of the second laser diode can be improved but also the height from the substrate surface of the first active layer can substantially accord with that of the second active layer. As a result, a difference in the height between light emitting parts of laser beams having different wavelengths can be suppressed.
The fourth method of fabricating a semiconductor laser diode array of this invention comprises the steps of forming a first cladding layer on a substrate from a first semiconductor of a first conductivity type; forming a first active layer on the first cladding layer from a second semiconductor; forming a second cladding layer on the first active layer from a third semiconductor of a second conductivity type; forming a fourth semiconductor layer of the second conductivity type on the second cladding layer; forming one part of a first semiconductor laser diode and forming a second laser diode formation region on the substrate by etching the first cladding layer, the first active layer, the second cladding layer and the fourth semiconductor layer with a first laser diode formation region on the fourth semiconductor layer masked; forming a height adjusting buffer layer on the second laser diode formation region from a fifth semiconductor of the first conductivity type; forming a third cladding layer on the height adjusting buffer layer from a sixth semiconductor of the first conductivity type; forming a second active layer on the third cladding layer from a seventh semiconductor having a larger energy gap than the first active layer; forming a fourth cladding layer on the second active layer from an eighth semiconductor of the second conductivity type; forming a ninth semiconductor layer of the second conductivity type on the fourth cladding layer; forming a fifth cladding layer from the fourth semiconductor layer and a sixth cladding layer from the ninth semiconductor layer respectively in the shape of ridges extending at an interval in parallel to each other by etching the fourth semiconductor layer and the ninth semiconductor layer; forming, on the second cladding layer on both sides of the fifth cladding layer and on the fourth cladding layer on both sides of the sixth cladding layer, a tenth semiconductor layer minimally absorbing light emitted from the first active layer and the second active layer; and forming the other part of the first semiconductor laser diode including a first current block layer and forming a second semiconductor laser diode including the height adjusting buffer layer, the third cladding layer, the second active layer, the fourth cladding layer, the sixth cladding layer and a second current block layer by forming the first current block layer on the second cladding layer from the tenth semiconductor layer and forming the second current block layer on the fourth cladding layer from the tenth semiconductor layer through etching of at least the tenth semiconductor layer with the first laser diode formation region and the second laser diode formation region on the tenth semiconductor layer masked.
In the fourth method of fabricating a semiconductor laser diode array, in addition to the characteristics of the third method of fabricating a semiconductor laser diode array, the fifth cladding layer and the sixth cladding layer can be simultaneously formed. Specifically, the fifth cladding layer and the sixth cladding layer in the shape of ridges extending at an interval in parallel to each other can be formed through one exposure process using one mask pattern having an opening pattern corresponding to these cladding layers. Therefore, a distance between these cladding layers can be determined depending upon the accuracy in the lithography. Furthermore, since the tenth semiconductor layer is formed on both sides of each of the fifth and sixth cladding layers, the first current block layer and the second current block layer can be formed from one semiconductor layer, resulting in simplifying the fabrication.
In the third or fourth method of fabricating a semiconductor laser diode array, a difference in a height from a substrate surface to a top surface between the first current block layer and the second current block layer is preferably approximately xc2x11 xcexcm or less.